In vivo studies of the simian sarcoma transforming gene in susceptible mice suggest unique biologic effects, distinctly different from other known oncogenes and including induction of long delayed, slowly growing sarcomas. These neoplasms are currently under intensive laboratory analysis. Transformation studies using human epithelioid cells infected with DNA and RNA tumor viruses have shown that at least two significant steps are needed to produce fully malignant transformation of normal epithelium. Collaborative studies on transplantability of human and animal tumors in athymic nude mice have furnished new data on tumor biology and malignant transformation.